In this chapter we discuss the qualitative difference between explicit knowledge and implicit knowledge (underlying mental representation). The chapter focuses on whether instruction affects the latter. We review the accepted finding that instruction does not affect ordered development. We also review the issue of whether instruction affects rate of development and ultimate attainment. We look at important variables in the research on instructed acquisition, including type of knowledge measured, the nature of assessments used in the research, and short-term vs. long-term studies, among others.